332 research outputs found
Shelling out for genomics
A report on the symposium 'Genomic and Proteomic Approaches to Crustacean Biology' held as part of the Society for Integrative and Comparative Biology 2006 Annual Meeting, Orlando, USA, 4-8 January 2006
Seawater acidification more than warming presents a challenge for two Antarctic macroalgal-associated amphipods
Elevated atmospheric pCO2 concentrations are triggering seawater pH reductions and seawater temperature increases along the western Antarctic Peninsula (WAP). These factors in combination have the potential to influence organisms in an antagonistic, additive, or synergistic manner. The amphipods Gondogeneia antarctica and Paradexamine fissicauda represent prominent members of macroalgal-associated mesograzer assemblages of the WAP. Our primary objective was to investigate amphipod behavioral and physiological responses to reduced seawater pH and elevated temperature to evaluate potential cascading ecological impacts. For 90 d, amphipods were exposed to combinations of seawater conditions based on present ambient (pH 8.0, 1.5°C) and predicted end-of-century conditions (pH 7.6, 3.5°C). We recorded survival, molt frequency, and macroalgal consumption rates as well as change in wet mass and proximate body composition (protein and lipid). Survival for both species declined significantly at reduced pH and co-varied with molt frequency. Consumption rates in G. antarctica were significantly higher at reduced pH and there was an additive pH-temperature effect on consumption rates in P. fissicauda. Body mass was reduced for G. antarctica at elevated temperature, but there was no significant effect of pH or temperature on body mass in P. fissicauda. Exposure to the pH or temperature levels tested did not induce significant changes in whole body biochemical composition of G. antarctica, but exposure to elevated temperature resulted in a significant increase in whole body protein content of P. fissicauda. Our study indicates that while elevated temperature causes sub-lethal impacts on both species of amphipods, reduced pH causes significant mortality
RXTE Spectral Observations of the 1996-97 Outburst of the Microquasar GRO J1655-40
Excellent coverage of the entire 16-month 1996-97 outburst cycle of GRO
J1655-40 was provided by RXTE. We present a full spectral analysis of these
data, which includes 52 PCA spectra from 2.5-20 keV and HEXTE spectra above 20
keV. We also include a nearly continuous ASM light curve with several intensity
measurements per day. The data are interpreted in the context of the multicolor
blackbody disk/power-law model. The source is observed in the very high,
high/soft, and low/hard outburst states. During the very high state, the source
exhibits intense hard flares on time scales of hours to days which are
correlated with changes in both the fitted temperature and radius of the inner
accretion disk. During the high/soft state, the spectrum is dominated by the
soft thermal emission from the accretion disk with spectral parameters that
suggest approximately constant inner disk radius and temperature. We find that
a tight relationship exists between the observed inner radius of the disk and
the flux in the power-law component. During intense hard flares, the inner disk
radius is observed to decrease by as much as a factor of three on a time scale
of days. The apparent decrease of the inner disk radius observed during the
flares may be due to the failure of the multicolor disk model caused by a
steepening of the radial temperature profile in the disk coupled with increased
spectral hardening and not physical changes of the inner disk radius. Assuming
that our spectral model is valid during periods of weak power-law emission, our
most likely value for the inner disk radius implies a* < 0.7. Such a low value
for the black hole angular momentum is inconsistent with the relativistic frame
dragging and the `diskoseismic' models as interpretations for the 300 Hz X-ray
QPO seen during some of these RXTE observations.Comment: 34 pages including 9 figures and 3 tables. Accepted for publication
in the Astrophysical Journal. Our interpretation of the data and the main
conclusions have been significantly revise
Passionate Scholars: Reforming Doctoral Education: A Research Project on Educational Outcomes and Processes
In this exploratory study, alumni of an innovative PhD program completed an open-ended self-administered questionnaire. They were asked to describe the intellectual, personal, and behavioral developments that they attributed to their graduate school experience, and aspects of their experience that they believed to have affected those changes. Findings indicated a wide array of changes in all areas
Applying the logic of sample surveys to qualitative case studies: The case cluster method
This is the publisher's version, also available electronically from "http://www.jstor.org"
The Low-Spin Black Hole in LMC X-3
Building upon a new dynamical model for the X-ray binary LMC X-3, we measure
the spin of its black hole primary via the continuum-fitting method. We
consider over one thousand thermal-state RXTE X-ray spectra of LMC X-3. Using a
large subset of these spectra, we constrain the spin parameter of the black
hole to be spin = 0.21(+0.18,-0.22), 90% confidence. Our estimate of the
uncertainty in spin takes into account a wide range of systematic errors. We
discuss evidence for a correlation between a black hole's spin and the
complexity of its X-ray spectrum.Comment: Submitted to ApJL, 5 pages emulateapj, 2 figures and 1 tabl
Inferring the Inclination of a Black Hole Accretion Disk from Observations of its Polarized Continuum Radiation
Spin parameters of stellar-mass black holes in X-ray binaries are currently
being estimated by fitting the X-ray continuum spectra of their accretion disk
emission. For this method, it is necessary to know the inclination of the
X-ray-producing inner region of the disk. Since the inner disk is expected to
be oriented perpendicular to the spin axis of the hole, the usual practice is
to assume that the black hole spin is aligned with the orbital angular momentum
vector of the binary, and to estimate the inclination of the latter from
ellipsoidal modulations in the light curve of the secondary star. We show that
the inclination of the disk can be inferred directly if we have both spectral
and polarization information on the disk radiation. The predicted degree of
polarization varies from 0% to 5% as the disk inclination changes from face-on
to edge-on. With current X-ray polarimetric techniques the polarization degree
of a typical bright X-ray binary could be measured to an accuracy of 0.1% by
observing the source for about 10 days. Such a measurement would constrain the
disk inclination to within a degree or two and would significantly improve the
reliability of black hole spin estimates. In addition, it would provide new
information on the tilt between the black hole spin axis and the orbital
rotation axis of the binary, which would constrain any velocity kicks
experienced by stellar-mass black holes during their formation.Comment: 46 pages, 8 figures, ApJ in pres
Modeling the Optical-X-ray Accretion Lag in LMC X-3: Insights Into Black-Hole Accretion Physics
The X-ray persistence and characteristically soft spectrum of the black hole
X-ray binary LMC X-3 make this source a touchstone for penetrating studies of
accretion physics. We analyze a rich, 10-year collection of optical/infrared
(OIR) time-series data in conjunction with all available contemporaneous X-ray
data collected by the ASM and PCA detectors aboard the Rossi X-ray Timing
Explorer. A cross-correlation analysis reveals an X-ray lag of ~2 weeks.
Motivated by this result, we develop a model that reproduces the complex OIR
light curves of LMC X-3. The model is comprised of three components of
emission: stellar light; accretion luminosity from the outer disk inferred from
the time-lagged X-ray emission; and light from the X-ray-heated star and outer
disk. Using the model, we filter a strong noise component out of the
ellipsoidal light curves and derive an improved orbital period for the system.
Concerning accretion physics, we find that the local viscous timescale in the
disk increases with the local mass accretion rate; this in turn implies that
the viscosity parameter alpha decreases with increasing luminosity. Finally, we
find that X-ray heating is a strong function of X-ray luminosity below ~50% of
the Eddington limit, while above this limit X-ray heating is heavily
suppressed. We ascribe this behavior to the strong dependence of the flaring in
the disk upon X-ray luminosity, concluding that for luminosities above ~50% of
Eddington, the star lies fully in the shadow of the disk.Comment: Accepted in ApJ (12 pages long in emulateapj format
Three-Dimensional Simulations of Magnetized Thin Accretion Disks around Black Holes: Stress in the Plunging Region
We describe three-dimensional general relativistic magnetohydrodynamic
simulations of a geometrically thin accretion disk around a non-spinning black
hole. The disk has a thickness over the radial range
. In steady state, the specific angular momentum profile of the
inflowing magnetized gas deviates by less than 2% from that of the standard
thin disk model of
Novikov & Thorne (1973). Also, the magnetic torque at the radius of the
innermost stable circular orbit (ISCO) is only of the inward flux of
angular momentum at this radius. Both results indicate that magnetic coupling
across the ISCO is relatively unimportant for geometrically thin disks.Comment: 4 pages, 4 figures, ApJL accepte
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